1. Summary of the Invention
The present invention relates to an ion implantation system, and more particularly, to a system of monitoring implant energy of an ion implantation device and method thereof.
2. Brief Discussion of the Related Art
In semiconductor fabrication, dopants are often applied to a semiconductor wafer to control a number of charged carriers and form a conductive area, using doping methods such as liquid deposition, thermal diffusion, or chemical evaporation. Ion implantation is used more widely due to its high precision.
During ion implantation, dopant atoms or molecules are first ionized, such as P+ or BF2+. Ions are accelerated by an accelerator to acquire a certain kinetic energy and then implanted into a semiconductor wafer. The depth distribution of the implanted ions is obtained by precisely controlling the output energy of the ion implantation device, with the dosage controlled by implantation time and current. Ion implantation provides uniform distribution, purity of dopants, and precise implantation areas with proper masks.
Typically, an ion beam comprises a plurality of ions of the same type and energy in a normal distribution. Output energy of an ion implantation process is typically controlled to 50 to 200 KeV. Depth distribution of the implanted ions in the resulting doped region varies with output energy of the ion implantation device. Due to inaccuracy, the real output energy, equal to the kinetic energy of the ions, is often different from the desired output energy of the ion implantation device. Thus, it is necessary to calibrate the ion implantation device for improved precision.
Ion implantation device calibration is typically performed by destructive procedures, with a test target, such as a silicon wafer, implanted with ions of predetermined output energy. Thereafter, the test target is cut and ion implantation conditions assessed by electron microscope, a complex and time-consuming procedure. Consequently, calibration cannot be performed frequently, and can be only performed for specific output energy levels, further limiting accuracy. Inaccuracy remains at about 200 eV after conventional calibration.
As semiconductor device dimensions decrease and integration increases, required doped regions move closer and closer to the surface of the semiconductor wafer, with ion implantation energy reducing accordingly, as low as 2 KeV. Despite implant energy decreasing significantly, inaccuracy of output energy from implantation device remains about 200 eV, an unacceptable level.